No it's not. Even if you want to count each NSMB game as 1-2-3, then NSMBU is NSMB4, not 3 (that'd be NSMB2). Don't forget NSMB Wii! ;)
Nintendo could totally release a NSMB3 if they wanted, regarldess of NSMBU. Naming convention is a bit over the place, though I doubt many people actually care anyway.
Right, I actually forgot about NSMB2. To me it's the most forgettable one, lol.
And that's true. They did a similar thing with Mario Kart.